Metallic structure within hightemperature furnaces



INVENTOF J. FLETCHER ETAL 2,446,222

STRUCTURES WITHIN MPERATURE FURNACES 3 Sheets-Shea?l 1 azi/;

METALLIC HIGH TE Aug. 3, 194g.

Filed July l0, 1942 James F/echer Cb lesLNoromJt? ATTO EY Aug. 3, 1948.

J. FLETCHER ET ALl METALLIC STRUCTURES WITHIN HIGH TEMPERATURE FURNACESFiled July 10, 1942 3 Sheets-Sheet 2 iNvEN-rons es F/ecber g3' le: LAforon, Jr:

ATTORNEY Aug 3, 1948.- J. FLETCHER ET AL Y 2,446,222

` LIC STRUCTURES WITHIN TEMPERATURE FURNACES Filed July 10, 1942 3Sheets-Sheet 5 INVENTUM James/etclzerf BY Char/QSL/Vortom AT u. EY

Patented Aug. 3, 1948 z,446,zz2

METALLIC STRUCTURE WITHIN HIGH- TEMPERATURE FUBNACES 1 A James Fletcher,Akron, Ohio, and Charles L. Norton, Jr.,l New York, N. Y., assigner-s toThe Babcock & Wilcox Company, Jersey City, N. J., a corporation of NewJersey Application .my 1o, 1942, serial No. 450,412

3 Claims. (Cl. 286-419) 1 The present invention relates to theconstruction of metallic structures adapted for use in a. corrosive oroxidizing atmosphere at temperatures approaching the melting point ofthe metal.

Ordinary iron or steel having a melting point of 27002900 F. oxidizescomparatively rapidly at temperatures above 930 F., and the oxidecoating formed does not protect the underlying metal from furtheroxidation. When heldfor apro-v longed period at high temperature in anatmosphere containing corrosive constituents, such as oxygen, carbondioxide, steam or sulphur dioxide. the metal is gradually converted tothe oxide and may entirely waste away much as it wastes away whenexposed to particularly corrosiveconditions at ordinary temperatures.According to some authorities, the rate of oxidation of iron at aconstant high temperature varies directly as the square root of theelapsed time. Due to this progressive scaling of the metal. ferrousstructures are ordinarily limited to use under service temperaturesbelow 900 F. For effective resistance to scaling at high temperatures,it has been universally considered necessary to use an alloy steel oriron.

Corrosion of ferrous metal structures at atmospheric or low temperaturescan be reduced in various ways, such as by the use of a coating ofprotective metal like zinc, tin, lead, nickel or copper; the applicationof protective paints; the production of oxide, phosphate or similarcoatings on the exposed surfaces; and passivating the surface. None ofthese protective methods however is suitable for service use aboverelatively low temperatures. For higher temperature service conditions,the structure to be protected can be calorizedto produce a coating ofaluminum and aluminum-iron alloys on the surface which tends to protectthe metal against high temperatures because of the formation of aluminumoxide on the surface. A calorized surface has been found to resistcontinued temperatures up to 1800" F. but begins to burn atapproximately 2000 F,

It has also been proposed to cover the otherwise exposed surface of thestructure with a layer of highly heat resistant alloy metals, either inthe form of a sheet welded to the exposed surface or a deposited layerof alloy weld metal. Such constructions however are expensive tomanufacture and require a substantial amount of alloy metal.

It` is also well known in the construction of water-cooled furnace wallsand the like to cover the furnace side of the water tubes lining thefurnace walls with a layer of initially plastic 2 refractory cement,such as a silicon carbide or a chrome ore cement, for the purpose oi'providing a heat radiating refractory surface to increase the furnacechamber temperature and in some cases to protect the tubes from erosionby slag depositing thereon. The refractory layer is usually held inplace by metallic projections, such as studs or ns, on the tubes whichalso serve to increase the rate of heat transmission to the tube wallsand contained uid. The rapid conduction of heat away from the tube wallsby the contained fluid holds the tube wall temperatures well below 930F., even with furnace chamber temperatures of 2500 F.-3000 F. andoperating pressures in excess of 2000 p. s. i. No special problem oftube metal oxidation is present under such conditions.

Where the metallic structure is to be subjected to internal or externalpressure stresses under service temperatures. alloy steels and specialheat-resisting alloys are universally used. in view of the rapidity withwhich carbon steels lose strength at high` temperatures. Another factorof importance is the lower permeability of alloy steels at hightemperatures to the infiltration of gases. The attention ofinvestigators therefore has been largely given to the properties ofalloy steels at high temperatures, and little is known of the strengthand permeability of ordinary iron and steel at temperatures above 1800F.

The general object of our invention is the provision of a metallicstructure adapted for prolonged service in a corrosive or oxidizingatmosphere at temperatures approaching the melting point of the metal. Afurther and more specific object is the provision of a protectivecovering for a pressure vessel made of a corrodible or oxidiz-`ticuiarity in the claims annexed to and forming' a part of thisspecification. For a better understanding of the invention, itsoperating advantages and specific objects attained by its use, refaremixed and formed into small briquets.

` arched roof Il containing gas outlets I2.

Of the drawings:

Fig. 1 is a sectional elevation of a retort furnace for the productionof metallic magnesium;

Fig. 2 is a partial longitudinal section of the furnace shown in Fig. 1;

Fig. 3 is a longitudinal section, partly broken away, of a furnaceretort constructed in accordance with our invention;

Fig. 4 is a transverse section taken on the line 4-4 of Fig. 3;

Fig. 5 is a fragmentary plan view of the expanded metal shown in Figs. 3and 4;

Fig. 6 is a view similar to Fig. 4 illustrating a modified retortlconstruction;

Fig. 'I is a sectional elevation of the lower part of a furnace baflieconstructed in accordance with our invention; and

Fig. 8 is a transverse section of a heat treating furnace containingfurnace rolls constructed in accordance with our invention.

While our invention is adapted for a wide field of use wherever astructure of a corrodible or oxidizable metal is subjected to corrosiveservice conditions at a temperature above that at which the bare metalcan safely be used, it is particularly designed for and especiallyuseful in the manufacture of retorts for the production of metallicmagnesium by the reduction of a magnesium-containing material.

In this process dolomitic limestone is burned and pulverized to a finelydivided condition; it is then mixed with a predetermined amount of apulverized reducing agent, preferably ferro-silicon. The pulverizeddolomite and ferro-silicon The briquets are charged so as to partly fillan externally heated sealed metallic retort which is kept under a highvacuum by means of a suitable vacuum pump. The charge in the retort isheated to a temperature of approximately 21002200 F.. i. e. Vabove thevaporization temperature of magnesium which is usually considered as1958- 2048" F. A chemical reaction occurs between the burned dolomiteand ferro-silicon resulting in the reduction of the magnesium oxide andthe formation of magnesium, calcium silicate, and iron, substantially inaccordance with the following reaction:

The operating pressure within the retort is -maintained as low aspossible, preferably at an absolute pressure of 0.1 mm. Hg. Such a highvacuum has been found to minimize wastage of the silicon by oxidation,formation of magnesium nitride, and discoloration and sponginess of thecondensed magnesium. The vaporized magnesium passes into the condensersection of the retort where it is recovered in a solid dense crystallinemass 99.98% pure. The condensed metal is removed from the retort,subsequently melted down under flux, and poured into ingots.

In Figs. 1 and 2 we have illustrated a furnace designed for thedescribed process of reductionv of magnesium ore. The furnace lll is ofsubstantially rectangular cross-section with an The furnace is heated byrows of gaseous fuel burners I3 oppositely arranged in the lower part ofthe side walls i4 and positioned between transverse refractorypartitions i5 extending the full width of the furnace. Short rebrickpiers Il are spaced along the top of each partition l5,A

tially the full width of the furnace chamber. Due to the high retortmetal temperatures in operation, the corrosive furnace atmosphere, andthe high vacuum maintained within the retort, it has been consideredessential to make the retort of stainless steel, such as an alloy of 25%chrome, 20% nickel, and the balance substantially all iron. Such alloymetal however, is expensive, and under war conditions relatively scarce.In accordance with our invention, such stainless steel retorts areeconomically replaced by retorts made of non-alloy ferrous metal havingavprotective covering preventing oxidation or corrosion of the retortmetal under the service conditions described.

In Figs. 3, 4, and 5, We have illustrated one form of our improvedretort, the body or shell of l which is formed by an extra heavy pipe ofmild steel. One end of the pipe is spun inwardly to form a rounded endportion 2'2 having a circular center opening which is closed by a metalplug 23 conforming in contour to the retort opening and welded thereinto form a gastight closure. The opposite or outer end of the retortshell is provided with an external peripheral flange 24 welded thereto.An end plate 25 is connected to the flange 24 in any suitable manner` toform a gas-tight detachable closure for the retort, adapted to beremoved for the charging and discharging of the retort. The outer endportion of the retort is constructed to form a magnesium vaporcondensing section by means of a Water/cooled circumferential jacket 26having water inlet and outlet connections 21 and 28 respectively, asindicated in Fig. 3. A cylindrical metallic sleeve 30 is arranged toclosely fit within the condenser section of the shell with its outer endsecured to the end plate 25 in any suitable manner. An outer section ofthe sleeve 30 is slotted as indicated at 3| to connect the interior ofthe shell to a short pipe 32 to which the vacuum pump (not shown) isdetachably connected. In operation the magnesium vapor condenses on thesleeve 30, which is removed with the end plate 25 at the end of thedistillation period and the condensed magnesium removed. A fresh chargeof briquets is then placed in the retort and the sleeve and end platerestored to their original position.

With a retort metal temperature of Z-2200 F., a non-alloy ferrous metalshell has been found to have a service life of only a relatively fewhours before the oxidation and scaling of themetal progresses to such an'extent that the desired vacuum could no longer be maintained or thethinning of the shell wall and pressure dinertial thereon caused theshell to collapse. In accordance with our invention the portion of theshell beyond the condenser section, i. e. the portion which normallyextends through the furnace side wall and into the furnace chamber isprovided With a protective covering which effectively prevents corrosionand scaling of the ferrous metal shell and which substantiallystrengthens the retort under the contemplated service conditions.

The preferred covering is preferably made of a highly refractorymaterial or materials, at least the portion of which contacting with theshell is chemically inert with respect to the ferrous metal shellthroughout the normal operating temperature range. Certain materials,such as silicon carbide cement, were found to alloy with the shell attemperatures of and above 2050 F. to form a layer of ferro-silicon whichprogressively penetrates the shell wall to a dangerous extent. Thematerial used in the outer portion of the protective covering must alsobe capable of forming and maintaining a substantially gas tight glazethroughout the exposed surface of the retort over a relatively widetemperature range. Coating the shell with the usual porcelain enamelglazes maturing at approximately 2100 F. was found unsatisfactory as theenamels were rendered porous after a relatively short heating period dueto the volatilization of some of theconstituents of the glaze. Othercoatings of known glaze forming materials were found unsatisfactory assome of the coatings did not flow or fuse while others bubbledexcessively and ran oif or cracked on drying and split olf from themetal. A further desirable characteristic for the covering material isthat at least the inner portion in contact with the metal shell shouldhave a thermal expansion rate commensurate with that of the shell metaland/or provisions be made for compensating for thermal expansiondifferences therebetween to avoid an undesirable amount of cracking ofthe covering material as the shell and covering expand or contract undervarying temperature conditions.

Another feature of our invention is the use of extended metallic surfaceon the outer surface of the shell in the form of studs, fins, orexpanded metal integrally connected to the shell. This extended surfaceserves several important functions, one being to provide a strongconnection between the shell and the contacting refractory layer. Thisnot only aids in holding the parts in their assembled relation but alsomaterially adds to the strength of the retort as a whole. As the highertemperatures are reached, the strength of the ferrous shell tends todecrease substantially while the strength of the refractory layerbecomes greater due to its vitriiication. The metallic projections onthe shell thus aid in forming a composite beam structure of the shelland refractory layer capable of withstanding th'e forces tending tocollapse the retort when operating under the described high vacuumconditions. Such' metallic projections are also useful in partlybreaking-up the continuity of the layer of contacting refractorymaterial and thus reducing the extent of any shrinkage cracks which maydevelop during the initial drying and vitrication of the refractorylayer as well as minimizing cracking of the refractory due to expansionand contraction ofthe parts. The parts will also serve to increase therate of heat transmission from the furnace chamber to the charge withinthe retort to a substantial extent.

The retortv protective covering illustrated in Figs. 3-5 is of acomposite type, being formed of an inner layer 35 of neutral refractorymaterial and an outer layer 36 of glaze-forming refractory material,withl the layers reinforced and held tightly together by suitable means.The retort shell is provided throughout. the area of the covering withstaggered rows of short steel studs 31 welded to theoutercircumferential and end sur-v faces of the shell. The studded area ofthe shell is th'en covered with a suitable moldable cement of initiallyplastic neutral refractory material rammed into place/so that it will betightly packed -against the shell and studs. manually with a smallheaded heavy hammer. Small areas are preferably applied at one time andthe previously applied areas kept moist to avoid any drying out. Theplastic refractory is This can be done portions of:

applied with a thickness such that it will completely cover the outerends of the studs.

We have found a number of plastic refactory cements which are inert withrespect to the metal ofthe retort throughoutthe operating temperaturerange and `which will vitrify but not fuse under such temperatureconditions. One highly suitable material is that known commercially as B& W Moldable consisting approximately of 80% crushed dense fire brick orgrog, 10% ball clay, 10% bond clay and a small amount of bind-y ingmaterial. Another highly suitable material is that known commercially asAlundu'rn cement which we believe consists of approximately 90% crushedfused alumina grain and 10% binder. This material appears to set harderand have a more permanent attachment to the shell and studs than mostother materials used. Another satisfactory material for the inner layerwas plastic chrome ore cement known commercially as PCO or KN andconsisting approximately of finely divided chrome ore, 5% ball clay, and10% sodium silicate. A further satisfactory refractory cementitiousmaterial was a mixture of 65% crushed re brick or grog and 35% fireclay.

It is usually desirable to clean and sand blast the shell outer surfacebefore applying the refractory to strengthen the attachment of therefractory layer. A thin coat of -corrosive resistant paint, such' aschrome paint, can also be applied underneath the refractory if desired.

In the service temperature range contemplated, the neutral refractorylayer alone would be insufficient to prevent oxidation and scaling ofthe retort shell and studs. The refractory layer has been found to bepervious and does not prevent the corrosive gaseous constituents of thefurnace atmosphere from penetrating the refractory layer and reachingthe retort metal. The shell and studs rapidly become pitted and wasteaway under the corrosive action. In accordance with our invention theouter portion of the covering is a layer of refractory material whichwill form a substantially gas-tight or impervious surface glaze or outerskin throughout the service temperature range contemplated over theentire exposed surface of the retort, and preferably a glaze which willbe semi-fluid, but not run oif or blister, at the normal operatingtemperature. We prefer to use a relatively thick coating of a materialwhich will form a smooth semi-fluid glaze on its exposed surface whilethe remainingenclosed portion will remain unglazed unless a pcrtion ofthe glaze is broken off or cracked whereupon the subjacent area thenexposed will glaze over and heal the break in the outer skin.

The most suitable glaze-forming material we have found having theforegoing characteristics is a silicon carbide refractory cement knowncommercially as "Carbofrax 5B and made from high grade silicon carbidegrain, raw clay and a soluble binder containing borax or other glazeforming constituents in the approximate pro 'Per cent' Silicon carbide84 Clay 12 Binder 4:

This cement is tempered with water to a suitable molding consistency. Itis believed that the silicon on the exposed surface will oxidize to SiOzand then combine with the borax in the binder to form a semi-plasticboro-silicate glassy skin on the exposed surface of the layer. Theremaining Carbofrax in the outer layer appears to remain as an unfusedamorphous structure unless the skin is ruptured in some manner whereuponthe rupture is-healed by the glazing of the newly exposed portion of theouter layer.

Another material which has been found. satisfactory for use as theglaze-forming outer layer is a refractory cement having a clay base inthe approximate proportions of Per cent Raw clay 28 Crushed heavy -rebrick'. 57 Dry sodium silicate 15 The dry materials are tempered withwater to a suitable molding consistency. The crushed re brick can bereplaced in part by silica sand.

In the manufacture of the retort illustrated in Figs. 3-5, the layer ofneutral refractory 35 is completely applied to the studded shell to thethickness indicated. We have found it desirable to provide meansforholding the refractory layer tightly against the shell and also tofacilitate the application of the outer refractory vlayer thereto.

For these purposes, a layer of wire mesh or expanded metal 38 of thecharacter shown in Fig. 5 is tightly wrapped around the retort andsecured in place. The expanded metal or metal lath may be in the form ofa single sheet wrapped around and the ends tack-welded or otherwisesecured together, or one or more narrow strips of such expanded metalwound around the refractory layer with a small overlap between turns.

yThe outer coating 36 was then applied over the `before or aftershipment to the point of use.

By way of example and not of limitation, one

magnesium furnace retort was made in accordance with the presentinvention with its shell 8' 51/2" long and of 10" ex. hy, seamlessforged steel pipe, the wall thickness being 1/2". Rows of studs diameteran-d long were welded on the shell with 11/2" staggered spacing. A SAthick layer of B & W 80 Moldable was rammed on the studs and shell.Metal lath shown in full size in Fig. 5 was then wrapped around theshell and a 1A" thick layer of Carbofrax 5B added. The retort was thenheated to 2300 F. and maintained at that temperature. The surface of theCarbofrax layer begins to glaze at about 1800 F. and remains in asemi-fluid con-dition throughout a temperature range up to and beyond2300 F. While the metal lath tends to melt and combine with silicon inthe outer layer to form a ferro-silicon, this action does not occuruntil the refractory materials are vtried to the desired extent. Theinert refractory layer effectively prevents any reaction between thesilicon of the outer llayer and the ferrous shell or studs, so thatthese metal parts are maintained intact and reinforced by thesurrounding vitried refractory to withstand the pressure stressesthereon `due to the high vacuum normally maintained within the retort,One advantage of the argillaceous base refractory cement outer layerdescribed is its inertness relative to the metal lath. Consequently theintervening layer of metal lath is advantageoisly maintained intact whensuch material is use Where the covered portion of the retort adjacentthe condenser Vsection is not likely to be heated to a glaze-formingtemperature under normal furnace operating conditions, it is desirableto preheat the covering in this section to a glaze-forming temperatureby a blow torch or other suitable means before installing the retort.Similarly, where the retort normal operating temperature is above 900 F.but below the glazeforming temperature, adequate protection isobtainable by preheating the retort to a temperature above theglaze-forming temperature to cause the yglaze to form.

In the modification illustratedin Fig. 6, the studs on the shell arereplaced by a layer of metal lath tack welded to the shell 2|. A layerof Alundum cement 4I is then rammed on the shell and over the metal lathand a secondy layer of metal lath 42 then wrapped around the Alundumlayer. An outer layer 43 of Carbofrax 5B is then added to complete theprotective covering, which is then heat treated as previously described.

While our protective covering has been specifically described inconnection with a retort of steel having circumferential ilns 66 ofstainless mild steel, it will be obvious that our invention isapplicable to other ferrous metals and alloys, such as cast iron 4andsteel, high carbon and alloy steels, etc. and to non-ferrous metals andalloys, such as tungsten and molybdenum, either to prevent corrosion orto increase their ability to withstand high temperatures. Nor is ourinvention limited to use in retorts or pressure vessels, as manystructural parts such as case hardening box-es, cyanide pots, crucibles,furnace supports and other furnace parts can be made in a similarmanner. Two of such uses are illustrated in Figs. 7 and 8.

In Fig. '7 -a furnace baille cone of the type disclosed in theThrockmorton et al. U. S. Patent 2,276,527 is shown with a refractoryand protected steel construction instead of the usual stainless alloymaterial. The lower end of the baille is formed by an hemisphericalhollow steel nose member supported by a hanger 5l and in turn supportingoverlapping circular rows of insulating lire brick 52 forming theremaining exposed part of the baille wall. The nose 50 has its otherwiseexposed surface studded as previously described and covered withsuccessive layers of` neutral plastic refractory, metal lath of expandedmetal or similar material and a glaze-forming refractory, as disclosedin Figs. 3-5.

In Fig. 8, we have shown a construction of a rotary supporting roll fora heat treating furnace embodying our invention. Such rolls arehorizontally arranged in external bearingsupports 6l and with a driveconnection 62. In operation the heating gases from the fuel burners 63flow down over the rolls and any material supported thereon, and escapethrough gas outlets 64 in the opposite wall. Instead of the usualstainless steel alloy construction, the roll can be formed 'by a hollowtube or shell 65 of mild steel alloy welded thereon at points spacedalong its length to directly support the articles to be heat treated.Between the fins, the otherwise exposed steel surface is studded andcovered with our protective covering 6l of neutral refractory, metallath of expanded metal or similar material and glaze-forming refractoryas illustrated. No

iluid cooling of the roll is necessary, as the protected shell iscapable of withstanding metal temperatures substantially above 1000 F.

While in accordance with the provisions of the statutes we haveillustrated and described herein the best forms of the invention nowknown to us, those skilled in the art willunderstand that changes may bemade in the form of the apparatus and composition of the materialsdisclosed without departing from the spirit of the invention covered byour claims, and that certain features of the invention may sometimes beused to advantage without a corresponding use o f other features. U

We claimz 1. A pressure vessel capable of use under co rosive conditionsat metal temperatures above 1000 F. which comprises a shell of metalsubject to rapid corrosion when exposed to 4said service conditions,metallic projections on the outer surface of said shell, and a heatresistant protective covering for said shell including an inner layer ofinitially plastic chemically in'ert refractory material on the outersurface of said shell and covering said projections, a layer of expandedmetal on the outer surface of said refractory layer. and an outer layerof metal' alloying glaze-forming material covering said inner layer andexpanded metal and forming'a self-sealing substantially gas-tight skinthereon.

2. A furnace retort capable of use under corrosive conditions at metaltemperatures above' 2000 F. which comprises a shell of non-alloy ferrousmetal subject .to rapid corrosion when exposed wto said serviceconditions, metallic projections welded to the outer surface of saidshell, and a heat resistant protective covering for said shell includingan inner layer of initially plastic chemically inert refractory materialcontacting 40 with the outer surface of said shell Vand covering saidprojections, a layer of pervious metal on the outer surface of saidrefractory layer, and an outer layer of glaze-forming material coveringsaid pervious metal layer and forming a selfsealing substantiallygas-tight skin thereon.

3. A metallic retort for the reduction of magnesium ore under a highvacuum and capable of use under corrosive conditions at metaltemperatures above 2000 F. which comprises a cylindrical shell ofnon-alloy ferrous metal subject to rapid corrosion when exposed to saidservice conditions, metallic projections welded to the outer surface ofsaid shell, and a heat resistant protective covering for said shellincluding an inner layer of initially plastic refractory materialneutral relative to said shell metal andA contacting with the outersurface of said shell and covering said projections, a layer of expandedmetal extending around the outer s'urface of said refractory layer, andan outer layer oi? initially plastic silicon carbide cement coveringsaid expanded metal layer and forming a self-sealing substantiallygas-tight skin thereon at temperatures above l800 F.

l JAMES FLETCHER..

CHARLES L. NQRTON, Ja.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS l Name

